The present invention relates to a soldering method using a solder with no lead contained, that is, a lead-free solder, and a joined object soldered with the use of the soldering method.
In view of environmental protection and since lead included in solder based on Snxe2x80x94Pb (tin-lead) used heretofore for fixing electronic components onto printed boards has a bad influence on not only the environment, but human bodies, solders not containing the lead, namely, lead-free solders have been under development lately. At present, the lead-free solders based on Snxe2x80x94Cu (tin-copper), Snxe2x80x94Ag (tin-silver), Snxe2x80x94Zn (tin-zinc), Snxe2x80x94Bi (tin-bismuth), Snxe2x80x94In (tin-indium), Inxe2x80x94Ag (indium-silver), etc. have been developed, and particularly the Snxe2x80x94Cu based, Snxe2x80x94Ag based and Snxe2x80x94Zn based types have potentialities.
However, in comparison with a melting point, i.e., 183xc2x0 C. of a conventional eutectic solder based on Snxe2x80x94Pb containing lead, a lead-free solder of a composition, for example, of Sn-0.7Cu in the Snxe2x80x94Cu has a melting point of 227xc2x0 C., the Snxe2x80x94Ag based lead-free solder in a composition of, e.g., Sn-3.5Ag has a melting point of 221xc2x0 C., and the Snxe2x80x94Zn based lead-free solder of a composition of, e.g., Sn-8Zn has a melting point of 199xc2x0 C. Although the melting point of the Snxe2x80x94Zn based type is lowest among these solders, since Zn is easy to oxidize and no effective means is found at present for preventing the oxidation, the Snxe2x80x94Zn based lead-free solder involves problems to be solved before used to fix electronic components onto a printed board as described hereinabove. Under the circumstances, the potential lead-free solder now is the Snxe2x80x94Cu based type and the Snxe2x80x94Ag based type, but both of which have higher melting points by approximately 40xc2x0 C. than that of the eutectic solder.
A heat resistant temperature of general electronic components is approximately 230xc2x0 C. Therefore, a thermal allowance of approximately 50xc2x0 C. is provided when the conventional eutectic solder is used to fix electronic components onto the printed board. However, the thermal allowance almost disappears in using, e.g., the Snxe2x80x94Cu based and Snxe2x80x94Ag based lead-free solders for the fixing. The thermal allowance is still more extreme for components weakly resistant to heat such as, for instance, aluminum electrolytic capacitors and the like.
For decreasing the melting points of the lead-free solders to or lower than the melting point of the conventional eutectic solder as much as possible, there is proposed lead-free solders in composition of, e.g., Sn-3.5Ag-6Bi, Sn-3.5Ag-3Bi-3In and the like having Bi (bismuth), In (indium) and the like added as a melting point decrease action metal acting to decrease the melting point.
The conventional eutectic solder changes almost instantaneously from a melt state to a solid state. On the other hand, the melting point of the lead-free solder decreases in proportion to an added amount of Bi when the Bi is added to the lead-free solder. But, when, for example, the Bi is added to the lead-free solder, a temperature range in which the lead-free solder changes from a melt state to a solid state expands as compared with that of the conventional eutectic solder. As a result, during solidification of the lead-free solder, the lead-free solder is brought into a state in which a partially solidified part and a part still in the melt state mix. Consequently, crystals of, e.g., Bi growing large in the lead-free solder 4 may segregation at a joining part 3 between an electronic component 1 and an electrode 2 of a printed board 5 as shown in FIG. 8. An enlarged illustration of the joining part in FIG. 8 diagrammatically shows a composition of the lead-free solder 4 at the joining part 3, in which a symbol xe2x80x9c◯xe2x80x9d corresponds to, e.g., Bi and a symbol xe2x80x9cxe2x96xa1xe2x80x9d corresponds to, e.g., Ag. A symbol xe2x80x9cxcex94xe2x80x9d shown at a joining interfacial part to the electrode 2 corresponds to a compound of Cu as a material of the electrode 2 and Sn in the lead-free solder.
Meanwhile, Bi itself has a higher hardness than hardnesses of Sn and Ag. Therefore a strength of the lead-free solder at a part where Bi gathers due to the segregation of Bi crystals becomes brittle when the Bi is included by, e.g., several tens wt. % in the lead-free solder. A joining strength at the joining interfacial part decreases if the Bi crystals are unevenly distributed and then solidified at the joining interfacial part of the electrode 2, leading to the trouble that a sufficient joining strength cannot be obtained between the electrode 2 and the electronic component 1. A content of Bi at present is hence unavoidably limited to several wt. % from a view point of a reliability on the joining strength, and therefore the melting point cannot be sufficiently lowered at present. Since the melting point of the present lead-free solder is higher than the melting point of the eutectic solder as depicted above, for example, a larger power is inevitably required for melting the solder than when the eutectic solder is used, thereby raising problems in view of costs and saving energy. Further components of a low thermal resistance cannot be soldered with the use of the lead-free solder.
The present invention is devised to solve the above-described problems and has for its object to provide a method of soldering a lead-free solder which lowers a melting point of the lead-free solder and prevents deterioration of a joining strength at parts joined by the lead-free solder, and joined objects soldered with the use of the soldering method.
In accomplishing the above and other aspects, there is provided according to a first aspect of the present invention a method of soldering a lead-free solder which comprises:
melting the lead-free solder which is an alloy of tin with no lead contained; and
acting ultrasonic vibration for increasing a joining strength between a mount article and a member to be mounted to at least one of the mount article and the member to be mounted which are to be joined by the lead-free solder, and the lead-free solder when solidifying the molten lead-free solder.
The ultrasonic vibration can be such that makes crystals of components contained in the lead-free solder fine and prevents segregation of the contained components, and increases the joining strength between the mount article and the member to be mounted.
Moreover, the ultrasonic vibration may be such that makes crystals of components contained in the lead-free solder fine and prevents segregation of the contained components at a joining interface of at least either the mount article or the member to be mounted, and increases the joining strength between the mount article and the member to be mounted at the joining interface.
The contained components may include a component of a melting point decrease action metal for acting to decrease a melting point of the lead-free solder.
In the case where the mount article and the member to be mounted contain Cu, the ultrasonic vibration may be such that increases a thickness of a layer of a compound of Sn included in the lead-free solder and the Cu, the compound existing at the joining interface, and increases the joining strength between the mount article and the member to be mounted at the joining interface.
The lead-free solder may have a Snxe2x80x94Ag based composition as a main ingredient.
When the lead-free solder has the Snxe2x80x94Ag based composition as the main ingredient, the above-mentioned contained components may include an alloy component of the Snxe2x80x94Ag.
The melting point decrease action metal can be at least one of Bi, Cu, Zn and In.
A joined object according to a second aspect of the present invention is soldered with the use of the method of soldering the lead-free solder of the first aspect.
According to the soldering method of the lead-free solder in the first aspect of the present invention, since the ultrasonic vibration is let to act when the molten lead-free solder is solidified, crystals of the components contained in the lead-free solder can be turned minute and prevented from segregating at the joining interface of at least one of the mount article and the member to be mounted which are to be joined by the lead-free solder, so that the joining strength between the mount article and the member to be mounted at the joining interface can be increased as compared with a case without the ultrasonic vibration acted.
In the joined object soldered with the use of the above soldering method according to the second aspect of the present invention, crystals of contained components of the lead-free solder are turned minute and prevented from segregating at the joining interfaces of the joined object as above, and the joining strength at the joining interfaces is increased as compared with the case where the ultrasonic vibration is not acted. The Joined object with a high joining strength can be provided accordingly.